The present invention relates generally to optical comb filters, and more particularly to high total transmission, tunable comb filter structures comprising moderately thick layers of optical material having periodic or multiply periodic refractive index modulation features resulting in a multiplicity of coupled, weakly-resonant optical cavities, characterized by spectra of high order relative to a fundamental resonance, consisting of narrow, moderate to high density reflection lines occurring in one or more sets, each set being characterized by lines equally spaced by wave number. Filters can be designed to be tuned, electrically or mechanically, such that the line peaks within a spectral band of interest shift by one harmonic order and/or from one peak position to the next. Thus a filter may be tuned to reflect or transmit light of any specific wavelength within a band. The individual features defining optical cavities may be thin compared to a wavelength of interest as, for example, when metal films are used as part of a singly periodic comb filter, or may consist of a section whose thickness is equal to several wavelengths.
For the purposes of describing the invention and defining the scope thereof, the term "optical" shall, in accordance with customary usage, be defined herein to include only vacuum ultraviolet, ultraviolet, visible, near infrared, mid-infrared and far infrared regions of the electromagnetic spectrum lying between about 0.01 and 1000 microns.
An optical comb filter may be defined as a filter whose spectra consists of a series of narrow reflection or transmission lines. In structures embodied by the present invention, the reflection spectra lines of a set are narrower then the spacing between neighboring lines; the spacing between line peaks of a set, in wave number units, is determined by the optical length of recurring optical cavities of equal optical length; cavities of equal length may be adjoining or separated and/or may physically overlap cavities of the same or of a different length.
The comb filter structures according to the present invention may yield sets of comb reflection lines whose peak amplitudes are uniform or modulated by one or more identifiable envelope functions relating to the detailed refractive index modulation of the material within the boundaries of the recurring optical cavities. The filters can be structured to produce envelope functions in accordance with application requirements.
There are numerous examples of prior art that are included under the above definition of a comb or tunable comb filter which are distinct from the present invention in design specifications and spectral characteristics. The best known example is a simple Fabry-Perot (F-P) filter, which consists of a single optical cavity with highly reflecting boundary features which may be metal films or index modulated dielectric films. The F-P filter transmits narrow special lines and reflects most light incident from a spectrally broad collimated source (see Atherton et al, "Tunable Fabry-Perot Filters", Opt Eng 20N;6, 806 (1981); Jenkins et al "Fundamentals of Optrics, 4th Edition, McGraw Hill, Inc. N.Y., N.Y. (1976); Born et al, "Principles of Optics", 3rd Revised Edition, Pergamon Press, N.Y., N.Y. (1965)). Interference filters based on thin film stacks and multiple F-P cavities have been described (Dobrowolski in Chapter 8 of the Optical Society of America, "Handbook of Optics) Mulitple F-P cavity films where the reflectors are moderate to highly reflecting metal films are discussed in Dobrowolski, but these are different in structure and function from the filters under consideration here. In those cases, cavities are F-P type and the spectra of interest are narrow transmission lines. This invention pertains to multiple cavity filters in which cavity resonances are relatively weak and which filters provide high order reflection comb lines and moderate to high total transmission of a broad spectral band.
When a broad band of collimated light is incident on a F-P cavity having low absorption, those frequencies which resonant within the cavity, the comb spectra, will be transmitted as narrow lines, while the majority of the light is reflected. Thus the total transmission of a F-P type comb filter is relatively low and the total reflectivity is relatively high, so long as the band of incident light is broad compared to the F-P filter spectral lines.
If an optical system requires high throughput of a broad band of optical frequencies, then a F-P type comb filter may be included only if used in reflection; Also, if high transmission of a narrow line source is of interest then the F-P filter may be suitable for providing high total transmission of such a source. If low total throughput is allowed for a band of frequencies then the F-P can be used to block light from most of a band while passing one or more narrow lines within the band (that, if F-P type filters can be used as moderate to low density reflection filters for bands of light between narrow ranges of transmission); however, if a high degree of rejection is required, as may be the case when using a tunable filter to reject a laser line, then the F-P is not a suitable tunable filter except when used in reflection.
The tunable comb filters which are the subject of this invention are distinctive is design and construction from the F-P type and capable of providing a function that is the opposite of that of the simple F-P type and those which are closely related. The filters consist of single or multiply periodic index modulation features forming cavities each of which is terminated by features presenting low to moderate reflectance of a more or less broad range of frequencies. Such a filter may highly reflect one or more sets of narrow lines, each line set being equally spaced in frequency or wave number (neglecting dispersion, i.e. n.sub.a =constant), while transmitting most of a relatively broad band of incident light. Such a filter can be used as a transmitting element in systems requiring high total throughput of a broad band of collimated light while providing a high degree of rejection of any specific in-band spectral line by tuning.
Another filter type conforming to the above definition of a comb filter, which is also distinct from the present design criteria, is a multiline, stack, rugate or Bragg type filter. These are not based on moderate to high harmonics of optical cavities in that the only significant reflections for a particular modulation are the first order (FIG. 4a, infra) and occasionally the second order. These filters are most typically constructed by stepwise or continuous modulation of the refractive index of a film in accordance with a profile resulting from superposing a number of individual periodic stepwise or sinusoidal modulations, each of which contributes to a specific spectral reflection line (see Gunning, U.S. Pat. No. 4,952,025). An alternative approach is to construct the multiline filter by laying down a single series of periodically modulated layers each having a different modulation period, such that each layer generate a specific reflection line (FIG. 4a, infra). These two types may be classified as parallel and series constructions, respectively. In at least one case it has been recognized that it may be advantageous to construct what might be called a parallel-series filter consisting of a superposition of two or more short periods with a long period of modulation extending through the entire structure wherein different parts of the total thickness contain different shorter periods of modulation, these parts occurring in series, as in Gunning. The presumed advantage is that the reflection lines will have a more nearly common optical density, since the optical density of each line of a filter is proportional to the number of periods of the corresponding part of the index modulation. The widths of the spectral lines for filters of this type are inherently broader than those of the cavity filters (see FIGS. 1, 4b and 5, infra) according to the invention, so that high total transmission of a broad band source cannot be maintained when lines are spaced close enough for practical electro-optical tuning from line to line.
A review of literature on tunable filters has not revealed any filters which are alike in design and function to those described herein. Yeh et al, "Electro-optic tunable filter structures," in Proceedings SPIE Vol 202 Active Optical Devices, p-1 (1979), Chang, "Electronically tunable optical spectral filters", Optical Engineering, Vol. 20, No. 6, p 805 (1981), and Gunning, "Electro-Optically tuned spectral filters: a review," Optical Eng. Vol. 20, No. 6, p. 837 (1981) review a number of types of tunable or cavity filters. Most are not designed to produce comb spectra, and when they are it is a line spectra in transmission separated by relatively broad regions of reflection. Some of the tunable filters deal with producing and or tuning one spectral line; for example, by deforming a thin film stack under high pressure to tune the fundamental (lowest order) reflection line (Kimura, et al. "Tunable multilayer-film distributed-Bragg-reflector filter", J. Appl. Phys., Vol. 50, No. 3, p-1222 (1979)), or inducing index modulation acoustically, (Chang, "Acousto-optic tunable filters, Optical Engineering", 20, No. 6, p-824 (1981);) Yariv et al, "Optical Waves in Crystals", John Wiley & Sons, N.Y., N.Y. (1984); or by rotating polarization between plane polarizers as with the electrical tuning of a stack of birefringent material (Henderson et al, "Programmable electro-optic tunable filters", SPIE Vol 202, Active Optical Devices, p. 16 (1979); Title et al, "Tunable birefringent networks", Ibid, p-47)). The filters that involve optical cavities exclusively deal with F-P type where cavities are bounded by moderate to highly reflecting structures which may be a metal film or a transparent dielectric film having a periodic refractive index modulation (Gunning et al, "Multiple-cavity infrared electro-optic tunable filter", Ibid p-21; van de Stade et al, "Multimirror Fabry-Perot Interferometers, Opt. Soc. Am." Vol. 2, No. 8, p 1363 (1985); Jain et al, "Dual tunable Fabry-Perot spectrally agile filter", Optical Engineering, Vol. 23, No. 2, p 159 (1984); and Maeda et al, "Electronically Tunable Liquid-Crystal-Etalon Filter for High-Density WDM Systems, IEEE Photonics Tech. Lett. 2, No. 11, p-820 (1990)). Some of the filters provide lines that are narrow in transmission which occur within a relatively broad reflection band associated with the optical period of the cavity boundary index modulation (Lytel et al, "Narrowband electrooptic tunable notch filter", Applied Optics, Vol. 25, No. 21, p-3889 (1986)). Some of the multiple cavity F-P type structures are concerned with the suppression of comb lines and/or increasing the free spectral range between transmission lines associated with a single F-P cavity, (Gunning et al, van de Stat et al, and Jain et al) while others are concerned with providing interference among cavities of slightly different length to broaden a transmission line while providing a sharp cutoff in transmission. (Dobrowolski, and van de Stadt). Others are concerned with controlling the strength of interference fields within a multiple cavity F-P structure in order to increase the transmission of a single narrow line by reducing absorption by defects at modulation steps. (Southwell et al, U.S. Pat. No. 4,956,682). Still others are concerned with employing nonlinear optical material in an F-P optical cavity to produce optical bistability. (Miller et al, U.S. Pat. No. 4,790,634). The construction of singly and multiply periodic cavity filters having narrow, moderate to high optical density reflection lines that provide moderate to high total transmission of a broad band and provide for rapid tuning of reflection lines over a free spectral range according to the invention is lacking in the prior art.
In accordance with a principal feature of the invention, so individual feature of a filter of the invention reflects more than 70% at any optical wavelength in a band of interest. The peak reflectance R(u,v) and the peak reflectance R and optical density D for the comb filters, respectively, corresponding to each individual element f(u,v) and group of like elements can be derived from the following set of relations, where absorption is assumed to be zero (Becker, "Design and Analysis of Optical Comb Filters", Tech. Rpt. WRDC-TR-90-4012., AD Number B142749 (1990) at pp 35, 40 and 59, and Yariv et al at page 197: EQU R(v)=1-T(v)=tan h.sup.2 .PSI.(v) EQU D(v)=-log.sub.10 (T(v)) EQU D(v)=2 log.sub.10 cos h.PSI.(v) EQU .PSI.(v)=U.PSI.(u,v).ident.bm(u,v).DELTA..epsilon./.epsilon..sub.a EQU D(v)=0.868.PSI.(V)-0.6, .PSI.(v)&gt;&gt;1 EQU D(v)=0.434.PSI..sup.2 (v), .PSI.(v)&lt;&lt;1
where b is a constant or order unity and equal to .pi./4 for sinusoidal modulation, T is the percent transmission, and D is the optical density, the other terms being defined below.
It is therefore a principal object of the invention to provide an improved tunable comb type interference filter.
It is a further object of the invention to provide a tunable comb filter such that reflection lines may be spaced at intervals that permit electro-optic tuning by at least one harmonic order, in order to reflect (prevent the transmission of) light of any specific frequency within a band of interest.
It is another object of the invention to provide a tunable comb filter providing multiple reflection lines such that each neighboring line pair associated with an optical cavity of length L.sub.R are separated by a spectral difference substantially larger (2 times or more) than the line half-widths in order that the total transmission of a broad light band may be moderate to high.
It is another object of the invention to provide a comb filter where the peak amplitude of individual or groups of reflection lines is weighted by control of the refractive index profile in addition to the refractive index excursion. In general such a filter may be described by a series of adjacent features each having a distinctive index modulation which is repeated at selected intervals, resulting in a series of optical cavities, which may overlap spatially and which provide for moderately to strong reflection lines as a result of the feature spacings being designed to provide constructive (in phase) reflections from a series of identical or similar features or cavities.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.